The difference is that if Doucoure/Allan were that bad every single game then fans would turn on them in an equal way. As it is, Allan has shown his class before, Doucoure hasn't been great but hasn't been constantly terrible either. So whilst they get highlighted...
... they don't get the venom a Sigurdsson/Iwobi get because that's the result of a 'dripping tap' effect, where the fans patience has long since ran out with them.
Iwobi isn't a scapegoat - he's legitimately god-awful. All the bloody time. All that is happening is people notice it and comment on it. The onus is on him to actually play football well, not on the fans to happy clap him.